This project proposes studies of HIV-seropositive individuals without AIDS who are at moderate risk for developing the ADC, along with a group of seronegative controls, in order to more precisely characterize the neurological and neuropsychological manifestations of this syndrome, as well as its immunological, virological and neuroradiological correlates, and its natural history. Clinical characterization will be accomplish by both cross-sectional and prospective/longitudinal studies to define the relative incidence, clinical presentation and evolution of the ADC. The neurological and neuropsychological data will then be correlated with systemic disease status, immunological function, virological findings in CSF and blood, and neuroradiological abnormalities including brain atrophy as and white matter changes revealed by magnetic resonance imaging (MRI). These studies will additionally contribute to refining neurological and neuropsychological instrumentation for evaluating the ADC. The data obtained in this Project will be combined with that derive from Projects 1 and 3, which examine individuals at different stages of HIV infection, in order to derive a broad picture of the evolution and natural history of the ADC.